Lewis-acidic metathesis polymerization

When well-defined, less Lewis-acidic metathesis polymerization catalysts are used to polymerize COT, a lower level of detectable sp defects are formed. Also, although the polyacetylene produced is still insoluble, the reaction proceeds slowly enough to allow manipulation of the liquid reaction solution before hardening. In this way, one can obtain films in a desired shape and location, e.g., on a semiconductor [123]. This procedure was found to result in better electrical contact than can be obtained when a free-standing film prepared via the Shirakawa route is simply pressed against an electrode. 

These 1987 resnlts concluded that classical metathesis catalyst systems were not sufficient and that Lewis acid cocatalyst-free systems were necessary if successM ADMET condensation polymerization were to become a reality. The key to snccessM ADMET polymerization was demonstrated " nsing the Lewis acid-free tungsten alkylidene metathesis catalyst (5a), the structure of which had been reported by Schrock et just one year earlier. When this... 

This book contains four chapters in which part of the recent development of the use of molecular rare-earth metal compounds in catalysis is covered. To keep the book within the given page limit, not all aspects could be reviewed in detail. For example, the use of molecular rare-earth metal complexes as Lewis acidic catalysts is not discussed in this book. The first two chapters review different catalytic conversions, namely the catalytic o-bond metathesis (Chapter by Reznichenko and Hultzsch) and the polymerization of 1,3-conjugated dienes (Chapter by Zhang et al.). Within these chapters, different catalytic systems and applications are discussed. The final two chapters are more concentrated on recent developments of... 

Reaction of [NF4][HF2] with a nonvolatile polymeric Lewis acid When, in the metathesis (1), all the materials except [NF4][XFg] are insoluble, product separation becomes impossible. This problem is avoided by digesting the Lewis acid in a large excess of [NF4][HF2] in HF solution, followed by thermal decomposition of the excess [NF4][HF2] at room temperature. 

The high oxophilicity of early transition metal catalysts (titanium, zirconium, or chromium) causes them to be poisoned by most functionalized olefins, particularly the commercially available polar comonomers. However, there are examples of copolymerizations with special substrates or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation. " Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation have been published.The cost of these multistep... 

The polymerization of oxanorbornene shown in equation (36) has been carried out using a series of different metathesis catalysts. Highly Lewis-acidic... 

The chemist of metathesis polymerization has been applied to preparation of unsaturated polycarbonates. This is a case of an acyclic diene metathesis. It takes place when Lewis-acid free catalysts are employed. An example of one such catalyst is Mo[CHC(CH3)2Ph](A 2,6-C6H3-/-Pt2)[OCCH3(CF3)2]2. One interesting point about this process is that unconjugated dienes are polymerized to high molecular weight linear polymers without formation of any cyclic structures. 

This hypothesis was supported by the reaction of WCl5/EtAlCl2 with styrene, which yielded polystyrene, presumably by cationic polymerization, rather than stilbene, the expected product of metathesis. The realization that vinyl addition competes with olefin metathesis in reactions using these classical catalyst systems was the key. This work serendipitously coincided with Schrock and coworkers report of their Lewis acid-free metathesis catalyst. These two advances, taken together, allowed ADMET to be realized as a method of forming a high polymer. 

The catalyst systems mentioned above are widely used in the commercial applications of metathesis polymerization due to their low costs and simplicity of preparation (see Section VII). However, the harsh reaction conditions and strong Lewis acids often required limit the utility of such catalysts [81]. These may cause side-reactions and make them incompatible with most functional groups [82]. The propagating species are poorly defined and often neither quantitatively formed nor uniform. Hence, there is often a lack of reaction control using these systems. Moreover, for the polymerization of functionalized monomers it is often necessary to use tin organyls instead of aluminium alkyls. These are more expensive and may cause severe injuries of health [25,83]. 

The first well-defined tungsten(Vl) alkylidenes which serve as highly active metathesis inititors were reported by Osborn and co-workers in 1982 [149]. W(=CH-t-Bu)(OCH2-t-Bu)2X2 (13) in combination with AlBrs or GaBrs polymerizes a variety of cycloolefins [61,62,149-151]. Later it has been reported that the related cyclopentylidene complex W(=C(CH2)4)(OCH2-t-Bu)2Br2 polymerizes numerous cycloolefins, e.g., various methoxycarbonyl derivatives of norbomene, even without addition of a Lewis acidic cocatalyst [152-154]. 

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